I did that for a while for winter bags; one 0 C down bag inside a -10 C synthetic fill bag. Warm in -35 C. But, and a very big but, once you are dealing with two sleeeping bags and one bivy (to keep the snow and sparks away) you are dealing with one more zipper, have one more bag that can (will) twist and turn during the night and when you place it inside the other.I would say yes for the nesting bags; interestingly: what about a down bag within a synthetic one? Anyone has some experience with that?
Works fine, subject to the usual caveats about keeping your down bag properly dry and aired. If it's not cold enough, you'll sweat too much and knacker the loft of the down.interestingly: what about a down bag within a synthetic one? Anyone has some experience with that?